1. Field of the Invention
The present invention relates to the field of toroidal inductive devices, and more particularly to toroidal inductive devices such as transformers, chokes, coils, ballasts, and the like.
2. Description of Related Art
Conventionally available toroidal inductive devices include a toroidal shaped magnetic core made of strips of grain oriented steel, continuous strips of alloys, or various powdered core arrangements, surrounded by a layer of electrical insulation. An electrical winding is wrapped around the core and distributed along the circumference of the core. This may be done in a toroidal winding machine, for example. Depending upon the type of toroidal inductive device, an additional layer of electrical insulation is wrapped around the electrical winding and a second electrical winding is wound on top of the additional insulation. An outer layer of insulation is typically added on top of the second winding to protect the second winding unless the toroidal device is potted in plastic or the like. A representative toroidal inductive device is described in U.S. Pat. No. 5,838,220.
Toroidal inductive devices provide several key advantages over the more common E-I type inductive devices. For instance, the magnetic core shape minimizes the amount of material required, thereby reducing the overall size and weight of the device. Since the windings are symmetrically spread over the entire magnetic core of the device, the wire lengths are relatively short, thus further contributing to the reduced size and weight of the device. Additional advantages include less flux leakage, less noise and heat, and in some applications higher reliability.
One significant shortcoming of conventional toroidal inductive devices is that the manufacturing costs far exceed those associated with the more common E-I type inductive devices. The costs are high because complex winding techniques are necessary to wind the electric windings around the toroidal shaped magnetic core.
An additional shortcoming of conventional toroidal inductive devices is that they have a vulnerability to high in-rush current. Conventionally available toroidal inductive devices generally cannot provide controllable magnetic reluctance, because they are generally manufactured such that they have no control over gap in a flux path. The gap provided is generally whatever space exists between the steel strips of the magnetic core. A resistor is often added in series with the primary winding of toroidal inductive devices to protect against in-rush currents. Some methods of creating gaps of desired sizes have been developed, such as the techniques disclosed in U.S. Pat. No. 6,243,940. However, those techniques, as well as others, only add to the costs of making the inductive device. Accordingly, conventional toroidal inductive devices and methods do not provide a cost effective way to create a desired gap size in order to accommodate in-rush currents.